A circuit interrupter is a disconnect switch used to periodically disconnect and reconnect an electrical power transmission, sub-transmission or distribution line from a connected device, such as a load, a line capacitor, a voltage regulator, or another type of device. Circuit interrupters typically include two or more contacts that are in physical contact with one another when the electric power line is connected to the switched device, and that are physically separated when the line is disconnected from the switched device. The interrupter is said to be in the "closed position" when the contacts are in contact with each other, and in the "open position" when the contacts are separated.
For an electric power line that carries high voltage and/or high current, it is desirable to open the male and female contacts quickly to avoid a restrike, in which the electric current arcs across a physical gap between the contacts. Restrikes impose high current spikes and serious voltage disturbances on the power line, and can also physically degrade the components of the interrupter, especially the contacts. These current spikes and voltage disturbances can also damage other pieces of equipment connected to the power line. Sensitive loads, such as computers and other electronic devices, are particularly vulnerable to this type of damage. Generally, the wider the arc gap during a restrike, the higher the voltage required to breakdown the gap, and the larger the current spike and the associated risk of damage.
Restrikes occur when the interrupter's contacts are not in physical contact, but are still close enough to each other to permit electric current to arc through the air or other media between the contacts. When the contacts of a properly designed circuit interrupter are fully separated, the distance between the contacts is sufficient to prohibit restrikes. However, a restrike can occur as the contacts are moved through an "opening stroke" from the fully connected or closed position to the fully separated position or open position. Likewise, an arc can occur across a gap between the contacts as the contacts are moved through a "closing stroke" from the open position to the closed position. However, arcs during a closing stroke are less dangerous to the electric system because the current in the circuit is zero prior to reconnection, which greatly reduces the current spike caused by the arc. Nevertheless, for safety reasons it may desirable to control the arcs during reconnection of the circuit interrupter.
Restrikes typically occur because once the circuit is opened at a zero voltage crossing, there is a rapid rise in voltage across the contacts known as the "transient recovery voltage." If the interrupter's contacts are not separated quickly enough for the gap between the contacts (the "arc gap") to withstand this rising voltage, then the gap breaks down and the current flow arcs across the gap and results in a restrike. A first restrike generally occurs at or near the point when the transient recovery voltage reaches its maximum value, which is typically one-quarter of a cycle from the zero voltage crossing when the circuit was initially opened. Thus, to prevent a restrike, the contacts must be moved from the closed position to a position at which a restrike is impossible within one-quarter of a cycle.
On an opening stroke in which the arc gap increases quickly, a second restrike is much more severe than the first because the arc gap is much larger. For this reason, in certain applications a maximum of one restrike is permitted. To meet this one-restrike-maximum, the contacts must be moved from the fully connected position to a position at which a restrike is impossible within three-quarters of a cycle. In particular, governmental regulations and municipal codes generally permit a maximum of one restrike per transmission or distribution line disconnection. Thus, the actuator mechanism of a typical interrupter must be capable of opening the contacts at a separation velocity sufficient to prevent multiple restrike (i.e., more than one) once the initial arc extinction occurs at a current zero.
Usually, a human operator of an interrupter cannot create enough energy to separate the contacts of an interrupter in a short enough time without a mechanical advantage. Thus, separation velocity is typically provided by an actuator mechanism, usually a spring arrangement, in the circuit interrupter. A typical spring arrangement stores potential energy in a spring-type actuator mechanism and then releases the spring energy abruptly to produce the desired separation velocity. Of course, higher separation velocity can often be accomplished by a more robust actuator mechanism. However, the designer of the circuit interrupter is also concerned with the cost and durability of the resulting device.
The designer therefore takes the intended use of the circuit interrupter into account when designing the circuit interrupter. For example, disconnection is often required to perform maintenance on the electrical power line or on a device connected to the line downstream from the disconnect switch, such as a transformer or voltage regulator. A disconnect capability may also be required for fault protection. A conventional circuit breaker is typically used as the circuit interrupter for these applications. In this application, the circuit breaker can be expected to cycle several dozen or a hundred or so times over its life span.
Line capacitor switches, on the other hand, can be expected to cycle much more frequently. This is because a line capacitor is typically switched into connection with the electric power line to correct the power factor during high-load periods. The line capacitor is later switched out of the circuit when the load drops and the power factor correction afforded by the capacitor is no longer needed. Because electric power loads typically peak on a daily or twice-daily basis, capacitor switches typically cycle on a daily or twice-daily basis. In addition, certain types of industrial loads, such as coal mines and arc furnaces, often impose peak loads many times each day. Therefore, a capacitor switch can be expected to cycle hundreds or thousands of times over its life span. A load switch, which is typically used to disconnect a discrete distribution voltage load such as customer-owned device or premises, may also experience hundreds or thousands of cycles over a lifetime.
In addition, it is economically feasible to design very expensive transmission voltage circuit breakers to provide fault protection for the transformer, which is a very expensive device. In addition, multiple restrikes at very high voltages can damage the transformer and other connected devices. Transmission voltage circuit breakers have therefore been designed with very robust actuator mechanisms, "penetrating contacts" (e.g., a male "pin" contact and a female "tulip" contact) that fit into each other, sealed chambers that surround the penetrating contacts with a dielectric gas that quenches the arcs within "arc gaps" between the contacts, and nozzles that direct the dielectric gas into the arc gaps as the penetrating contacts separate. Although these features are very effective at minimizing restrikes, they have traditionally been too expensive to be feasible for inclusion in sub-transmission and distribution voltage devices, such as capacitor and load switches.
Conventional circuit breakers have a number of other attributes that make them unsuitable as capacitor or load switches. First and most importantly, circuit breakers are not designed to withstand the hundreds or thousands of cycles that capacitor and load switches must withstand. For example, circuit breakers typically include "stop" mechanisms for charging and then abruptly releasing spring energy. These stop mechanisms are prone to wear and tear and thus limit the durability of the circuit breaker. Bellows placed around high-speed actuators to seal the dielectric gas chambers are also prone to wear and tear through repetitive cycling of the breaker. A circuit breaker would therefore break down far to quickly to be cost effective if used as a capacitor switch. Second, circuit breakers are normally operated as series-connected devices, which raises their cost as compared to disconnect switches that are normally disconnected from the circuit and only conduct current when temporarily connected during disconnect operations. Third, circuit breakers typically include separate actuator mechanisms for opening and closing the breaker, which also raises their cost as compared to a disconnect switch that includes a single actuator mechanism.
Electric switchgear manufacturers have developed circuit interrupters for sub-transmission and distribution applications that overcome some of these disadvantages. For example, normally disconnected circuit interrupters have been developed for use as capacitor and load switches. However, these devices are not designed to prevent restrikes, but instead include a series connected cascade of sacrificial "butt" contacts that are designed to deteriorate over time as a result of restrikes. The deterioration of the contacts requires regular maintenance to monitor and replace the contacts as they deteriorate, and thus increases the cost of using this type of circuit interrupter. These devices are also prone to cascading failures when one of the butt contacts deteriorates to the point of malfunction. These circuit interrupters are also designed to control the arc only on the opening stroke, and typically conduct an uncontrolled arc through air on the closing stroke.
Although transmission voltage circuit breakers are available with penetrating contacts, dielectric gas chambers, and actuators that accelerate the penetrating contacts to quench arcs between the contacts within the dielectric chambers during circuit opening, these features are not presently available in sub-transmission or distribution devices, such as capacitor and load switches. Moreover, circuit breakers with these features are not presently designed to be economical enough to serve as capacitor or load switches. Available capacitor and load switches, on the other hand, are not presently designed to avoid multiple restrikes or to accelerate their contacts to control the resulting arcs on both the opening and closing strokes. The limited durability of conventional capacitor switches with sacrificial contacts also limits their feasibility for many applications.
Therefore, there is a need for a circuit interrupter that prevents or limits restrikes, and that is durable enough to be used as a capacitor and load switch. There is a further need for a normally disconnected capacitor switch that controls the arc on both the opening and closing strokes. There is also a need for more durable and cost effective capacitor and load switch designs.